The present invention generally relates to the systems for laying a cable into a guide tube and is concerned more particularly with the laying of a cable, such as a fiber optics and the like, into a guide tube utilizing a hydraulic fluid, such as water and the like, as the main means for the flotation, the transport and/or the laying of the cable- As is known, the traditional laying technique provides that the cables suitably protected are layed after the excavation of an "opencast" trench. This involves, besides a vast amount of work with the consequent labour employment, also a high cost: indeed, it has been estimated that, in a cable telephone installation, civil work, such as excavations, embankments, rehabilitations, etc., weighs upon the total costs even in an amount of the 70% of the total cost.

Within the frame of a global solution to these problems, in particular with the advent and the more and more spread utilization of the fiber optics cables, one is providing for preliminarily installing plastics preformed tubes, into one of which a first cable is inserted, while the remaining empty tubes are intended to shelter future cables, without further excavations and additional work and relevant expenses.

With so realized infrastructures, the operation for laying a new cable will be in practice

an insertion or, as said with a neologism now widespread in the field of this technique, "threading" operation.

The threading, as one deduces from the term, consists in inserting a cable into an already layed tube and in pulling it by , one of its extremities. In practice one operates as follows: one prearranges a pulling rope in the tube, which rope is pulled by a motor winch generally with the control of the speed, of the maximum allowable force and of other parameters useful to render the operation safe. At the extremity of the rope, by the side opposite the winch, the cable is hooked that is pulled by the rope and runs at the interior of the tube for al the length thereof. Naturally, the longer the cable and the more tortuous the seat of the tube, the more difficult this operation .

It is recalled, in this connection, that the length of the old copper cables was of a maximum of 500 m, while, with the fiber optics cables utilized at the present day, the ■ usual length is of 3000 m and the tendency for the future is of arriving at cable 1 engths of 6000 m.

To recognize the problems inherent to this technique, the following mathematical analysis is set forth.

In the case of a tube with a route straight upon the vertical plane and upon the vertical one, the pulling force is: = uPl where: u = the friction coefficient between the cable and the tubing,

P = The specific weight of the cable,

1 = the length of the cable.

In the case of a tube with a curved route, with some approximations, the pulling force is expressed by the reduced formula:

0.175θu

F _£ = F.e where :

F = the pulling force after the curve, F = the force before the curve, θ = the curve angle, u = the friction coefficient between the cable and the tubing.

As is seen, when one has curves in the route of the tube, which curves can lay upon both the vertical plane and the horizontal plane, and this is the normal condition, the pulling force increases exponentially. While the friction coefficient is normally reduced by utilizing special lubricants (which are very expensive), the pulling force can assume, owing to its behaviour, such values as to make all the operation to fail.

A practical system normally employed to reduce the drawbacks of the laying, is that of laying half the length of the cable by a side and half by the other side. This involves a preliminary unwinding of the second half of the recovery bobbin with an expensive and delicated handling of the same.

The general object of the present invention is to suggest a process and the apparatus for realizing the same, such as to avoid the drawbacks of the systems of the known technique.

A first aspect of the invention, therefore,

is concerned with a process for the threading of a cable into a tube by means of a pressurized hydraulic fluid in which the following operations are performed:

- feeding the cable, that is to be threaded, from a controlled speed bobbin; advancing said cable towards the tube at a speed as much uniform as possible;

- inserting an end of the cable into a first extremity of the tube; - simultaneously introducing into said first extremity and pushing through said tube a hydraulic fluid in such a manner that said hydraulic fluid exerts a flotation force upon the mass of said cable and an entrainment force such as to bring the cable up to the outlet by a second extremity of the tube.

A second aspect of the invention is concerned with an apparatus for carrying out the aforementioned process, comprising:

- a cable holder bobbin actuated to dispense the cable, that is to be inserted, by a variable speed oleodynamic motor; a device for controlling the speed of rotation of the bobbin, comprising a tension sensor associated with the cable which is unwound and dispensed by the bobbin; a controller device for adjusting the slide speed and therefore the threading speed of the cable into the tube by accelerating and/or decelerating the cable according to the need in such a manner as to avoid sudden accelerations/decelerations thereof and consequent possible water hammerings;

- a flow chamber device hermetically connected

with the inlet extremity of the tube to simultaneously canalize into ' it the cable that is to be threaded and the flotation/ entrainment hydraulic fluid.

Further particularities and advantages of the present invention will turn out to be evident by the continuation of the disclosure with reference to the annexed drawings in which the preferred embodiment of the apparatus is represented as a matter of ilustration and not of limitation, and in which: Figure 1 shows a side vertical assembly view of the apparatus for threading a cable into a conduit by means of the procedure of the present invention ;

Figure 2 shows a perspective view of the sections of flow chamber and controller device of the apparatus of Figure 1;

Figure 3 shows a view sectioned horizontally at the height of the cable of the controller device;

Figure 4 shows a view sectioned vertically of the controller device;

Figure 5 shows a vertical section view of the slide or flow chamber;

Figure 6 shows a horizontal section view of the slide or flow chamber; Figure 7 is a side elevation view of a pre¬ ferred arrangement of a first station of the equipment to carry out the method of this invention;

Figure 8 is a side elevation view of a pre¬ ferred arrangement of a second station of the equipment to carry out the method of this invention.

With reference now to the drawings, it is seen that the apparatus for carrying out the threading

process according to the present invention consists of three main sections: a flow or slide chamber 10, ^* a controller device 11 and a cable dispenser assembly 12 with an annexed speed controller device and block 13. Auxiliary service apparatuses, such as hydraulic motors, compressors, pumps, etc. are annexed to these main sections.

The fundamental section is that of the flow or slide chamber 10, represented in detail in Figures 5 and 6. Such a chamber comprises an internal room C and presents an outlet port and one or more inlet ports. To the outlet port the free extremity of the tube or conduit 14 into which the cable 15 is to be threaded is sealingly connected. The sealing is ensured by a suitable ring packing 16. In the execution of the figures the chamber 10 presents two inlet ports connected with two tubes 17, 18 for the pressurized introduction of a hydraulic fluid, preferably water. As already mentioned and as will be analyzed in the following, it is this hydraulic fluid that in its flow through the conduit 1.4 will entrain the cable 15, introduced into the chamber through a suitable diameter adapter 19, pressure tightly, exerting upon it a flotation thrust also that will tend to maintain it raised from the walls of the conduit 14 and "floating" in the conduit itself.

A drive station imparts upon the fluid a pressure energy sufficient to overcome the piezometric height of the extremity far from the tube or conduit 14 (i.e. to get out the fluid by the other extremity), such a flow rate that the "outflow" coefficient is close to unity (i.e. one operates under a penstock condition, such that in the conduit 14 fluid empty

zones do not remain and the inlet flow rate is the same as the outlet flow rate) and, finally, a momentum suitable to realize the entrainment of the cable.

The flow chamber system has such a configuration as to realize the same conception of

Venturi 's meter, as is apparent in Figures 5 and 5, so canalizing the cable and the fluid in the fitting for acessing the tube under the most suitable condition.

Under these conditions, the speed of the fluid in the hydraulic circuit is therefore greater than the one of the laying of the cable and its value is computed on the basis of the entrainment energy necessary for each type of cable. As already said, upon the cable a combined action is realized of the flotation thrust and of the entrainment thrust, both due to the flow of the hydraulic fluid that moves in the same direction of the laying of the cable. This combined action acts upon the surface of the cable for all its length in a distributed and uniform manner.

The effect and the result of this distribution and uniformity leads back the problem, from a theoretical point of view, to that of laying a cable into a straight and plane conduit. At this point, therefore, all those forces disappear having an exponential variation that concentrate in all tube layouts and in particular in the difficult and singular layouts.

Another advantage that is obtained is that o ^f not having the maximum tractive effort on the head of the rope when the cable is wholly pulled by an extremity. Tha latter effect, indeed, is reductive

and limits the length of construction of the cables in the mill.

With the present invention, wherein the cable in a way floats on the fluid or at least receives a vertical thrust for its flotation in it, and neither longer the need for employing any lubricant to reduce the contact friction between the cable surface and the tube surface exists.

AS regards the hydraulic circuit, it is provided that it can be closed, i.e. that the fluid outgoing from the extremity far from the conduit 14 can be recovered and sent back to a central reservoir to be put again into circulation. Moreover, in correspondence with the access, the output and the eventual discontinuity points of the laying tube, plastics tubular fittings can be used so reinforced as to bear internal pressure and transparent to visualize in a way the internal state of the fluid and of the cable entrained in it. A second important section of the apparatus according to the present invention is the section of the controller device 11 represented generically in Figure 2 e in detail in Figures 3 and 4. As is seen, it consists of two track members actuated by a hydraulic motor 20, cooperating in such a manner as to tighten the cable 15 with a strong friction between the two opposed longerons and sliding in the same sense and entrained by two entrainment/drive toothed wheel trains. The function of the controller device is that of rendering the speed of slide of the cable 15, and therefore the laying speed, as much uniform as possible, avoiding sudden variations

of such a speed. Indeed, by virtue of the track structure with a strong friction force between tracks and cable, it is able to accelerate and/or braking the cable, according to need, so as to essentially avoid eventual water hammerings or anyhow eventual dangerous recoils recurring in penstocks of a certain length for the transport of fluids.

The third section of the apparatus is the section 12 for dispensing the cable, which comprises a bobbin 20 actuated by a motor oleodynamical system 21, having a variable speed, which rotates the bobbin 20 with the speed required by the laying.

AS the linear velocity of thye cable has to be as constant as possible also against the variation of the diameter of the winding, a lever mechanism 13 is provided operating as a sensor of the tension of the cable 15, the output signal whereof drives the motor oleodynamical system and after all acts upon the rotation speed of the unwinding bobbin 20.

Referring now to Figs. 7 and 8 a preferred arrangement of the equipment to carry out the method of this invention is shown.

The equipment is comprised of a first and of a second stations. The first station is the driving station and is aimed at threading and forwarding the cable. This station is very light and is mounted upon a mobile frame 110. A castor wheel 111 and the ground legs 112 and 113 are shown. A flow chamber unit 114, the operation of which has already been described, is mount¬ ed upon the upper table of the frame 110. The flow chamber unit 114 is adjustable as to height by means of a lead screw 115 with a handwheel 116. The planarity of

the flow chamber unit 114 is adjusted by adjustement screws 117, 118.

High pressure water supply tube 119 is connec¬ ted from the supply section to the flow chamber unit 114.

The cable to be threaded is supplied to the flow chamber unit 114 by means of a controller unit 120 designed to adjust and regulate the driving speed and consisting of two track members with a lead screw 121 and a control handwheel 122.

Two cable guiding roller assemblies 123 and 124 are arranged upstream of the controller unit 120. Each roller assembly includes two roller pairs arranged with mutually orthogonal axis. A control board (not shown) is also part of this section.

The second station of the equipment as shown in Fig. 8 substantially includes a internal combustion engine 125 with automatic starter, by which the water pumping section 126 as well as other hydraulic units are powered. The pressure water supply tube 119 is connected to the outlet of said water pumping section 126.

The second station is a supply station and also comprises the water supply tank 127 and a mobile container for the cable coil 128, which is operated by means of a chain belt drive housed in a shaped case 129.

A grear reduction unit is designated with 130 and a tank for the hydraulic oil needed for the opera¬ tion of the pump unit 126 is shown with 131. Further components are a hydraulic pump 132 for driving the cable coil 128 and a winding-up device 133 for a self-contained water feeding hose.

In order to better understand the invention,

a mathematical analysis of the parameters and of the magnitudes at hand will be now set forth. Basic Principles

The fundamental quantities that characterize the flotation system of the present invention, considering a tube element of one meter, water as the fluid and a fiber-optics cable weighing 350 g/m, are :

1) FLOTATION THRUST

x1x10 ³ = 0.314 kg where: D = 20 mm, the diameter of the cable,

3 J = 1000 kg/m , the specific weight of water.

2) FLUID SPEED The pulling force is:

£ = u x P x 1

= 0.1 x 0.35 x 1 = 0.035 kg The thrust of the fluidon the cable is: = q(V - C) = j> x V(V - C) 0.035 = 102 x 78.5 x 1θ ^"4 x V x (V - 0.5)

V ² - 0.5V - 0.044 = 0 THe real solution is: V = 0.58 m/s where : u = 0.1 , the residual friction coefficient between the surface of the cable and the tube,

P = the specific weight of the cable, o = 102, the density of water, q = the flow rate of the fluid, V = the velocity of the fluid,

C = 0.5 m/s, the laying velocity,

— Fς

R x R x 3.14, the section reacting c t

aganist the thrust,

R = 10 mm, the radius of the cable, c

R = 25 mm, the radius of the tube.

3) FLUID FLOW RATE The flow rate under non turbulent laminar flow conditions is: q_ = AV

= 78.5x10 ^"4 x 0.58 = 45.53 x 10~ ⁴ m ³ /s = 4.55 1/s, where: A = the vertical section of the tube.

4) THRUST ON THE CABLE

The thrust of the nappe on the cable is: £ _s = .oq(V - C)

= 102 x 45.53 x 10 ^°4 x (0.58 - 0.5) = 0.0372 kg.

5) HEAD LOSS

The head loss for a penstock with the exclusive service of extremity with a single diameter is :

= 2.02 x 10 ^_3 (45.53 x 1θ ^"4 )/3.125 x 1θ ^"7 = 0.13 m/m where :

B = roughness coefficient in Darcy's formula, 3 = 1.26 x 1θ ^"3 + 38 x 10 ^_6 /D

= 2.02 x 10 D = 50 mm, the diameter of the tube. 6) DRIVING POWER The necessary power, considering the tube in a plane , is

W = Jq[V ² /2g + C ² /2g + ∑Y] = 0.1365 + 4.55Y kgm/s = 6.1 x 10 ^"2 x Y CV

where g = gravity acceleration

∑Y = summation of the head losses of the tube.

RESULTS

With the flotation and entrainment system by means of a fluid it happens that:

- it is possible to lay a cable length of 6000 m with a single apparatus by a single side with a solution of continuity,

- it is possible to achieve an average laying speed of about 30 m/min,

- it isn't necessary normally to synchronize the different apparatuses that intervene in the roanufacture , it is possible to eliminate the pulling forces of exponential nature,

- it is possible to obtain an entrainment force for laying distributed along the cable. In conclusion, resuming, the system according to the present invention:

- avoids the laying of the rope for pulling the cable; avoids the singular distributed and/or concentrated overload tensions during the operations for laying the cable;

- allows the laying of the cable also in "difficult" runs;

- allows the laying of cables of any size length;

- can lower the fabrication costs of fiber optics cables, reducing the quantity of Kevlar, used

at the present day for the mechanical protection of the pulling effort.

All the process has been disclosed with reference to a purely exemplifying embodiment, those skilled in the art being evidently able to conceive variations that allow the same end to be reached, by exploiting the same principles, without so departing from the scope of protection of the present invention.